This invention relates to the growth of diamond under conditions of high temperature and high pressure.
The synthesis of crystals at high temperatures and high pressures, particularly diamond, is very well established. There are two principle methods employed, both from solution, namely a temperature gradient method and an allotropic change method. In the temperature gradient method, the driving force for crystal growth is the supersaturation due to the difference in solubilities of the source material and the growing crystal as the result of a temperature difference between the two. In the allotropic change method, the driving force for crystal growth is the supersaturation due to the difference in solubilities of the source material and the growing crystal as the result of an allotropic (or polymorphic) difference between the two.
According to the present invention, a method of growing diamond crystals includes the steps of providing a source of diamond crystals, providing a plurality of growth centers defined by diamond crystals. The quantity of source crystals generally being greater than that of the growth centers, producing a reaction mass by bringing the source and growth centers into contact with a solvent/catalyst, subjecting the reaction mass to conditions of elevated temperature and pressure suitable for crystal growth. The necessary supersaturation of carbon in the solvent/catalyst being achieved, at least in part, and preferably predominantly, by a selection of particle size difference between the source crystal and the growth centers, and recovering the diamond crystals from the reaction mass.
The invention allows for the growth of diamond crystals using supersaturation created, at least in part, by a difference in particle size between the source crystals and the growth centers. The source crystals and growth centers may be provided by particles at opposite ends of a particle size range. Thus, in this case, the growth centers will be provided by crystals at the higher end of the particle size range, whilst the source crystals will be provided at the lower end of the particle size range. The quantity of crystals, i.e. number of crystals, at the lower end of the range will generally be much greater than that at the higher end of the range.
The source crystals will be smaller than the growth centers. The size of the source crystals will thus depend on the size of the growth centers. It has been found that particularly good diamond growth is achieved if the source crystals have a size of less than 20 microns and generally less than 15 microns.
The growth centers may also be provided by seed crystals which are separate and distinct from the source crystals. Such seed crystals will generally be substantially larger than the source crystals. An example of this form of the invention is to use particles of a size less than 10 microns as source crystal particles and seed crystals having a size substantially greater than 10 microns as growth centers. The seed crystals may, for example, be greater than 40 microns. The quantity of source crystals will again generally be much greater than that of the seed crystals.
The method of the invention, it has been found, produces a mass of diamond crystals in which at least 40 percent, typically at least 80 percent, and generally substantially the entire mass, consists of synthetic twinned diamonds. The twinned diamonds include contact twins, macle twins, including multiple and single made twins, polysynthetic twins and star twins. Various shapes of twin diamonds are produced and these include blocky or cube shape, plate shape and column shape. For the plate and column shaped diamonds, the crystals have a high aspect ratio, i.e. a high ratio of longest dimension to shortest dimension. A mass of twinned diamond crystals of this nature is believed to be new and forms another aspect of the invention.